Seismology of δ Scuti stars in the Praesepe cluster I. Ranges of unstable modes as predicted by linear analysis versus observations

We consider here a representative set of δ Scuti stars observed in the Praesepe cluster, thanks to recent efforts of multi-site networks. We show that the effect of rotation on the determination of fundamental parameters Mbol and Teff cannot be neglected when modelling fast rotators as δ Scuti stars. We propose a method to take this effect into account and gain some indication on the rotation rate of the stars. We then use this unique homogeneous set of observational data to investigate the pulsational behaviour of δ Scuti stars for different masses, corresponding to different evolutionary stages at the age of the Praesepe cluster. We propose here an alternative approach to seismology of δ Scuti stars, trying to avoid specific but questionable mode identifications. Via a large set of models, we compare the observed modes with the unstable modes as obtained from a linear stability analysis including a nonlocal time-dependent treatment of convection. We show that, in the framework of the linear analysis adopted here, a satisfying agreement can be reached between the predicted ranges of unstable modes and the ranges derived from observations. However we found that for “low-mass” δ Scuti stars (1.6-1.80M ) the agreement is found only for a restricted range of values of the mixing length parameter. These stars thus bring a valuable observational constraint on the description of the convection in the outer layers of stars.